Three dimensional quadrupole ion traps (e.g., 3-D ion traps) are commercially available devices used as mass spectrometers. A 3-D ion trap can be used as single mass analyzer or as a tandem mass spectrometer. A linear quadrupole (e.g., 2-D ion trap) is another commercially available quadrupole device that can be used as a mass analyzer, and/or an ion storage component and/or as an ion collision cell for a tandem mass spectrometer. Typically, ions and/or molecules are introduced in both the 3-D and the ion trap 2-D via an aperture.
In both the 3-D ion trap and the linear quadrupole, the presence of the aperture inevitably introduces some deviation into the quadrupole field (e.g., an ideal quadrupole potential no longer exists). This deviation often negatively impacts the performance of the quadrupole. For example, the deviation may cause peak splitting, mass shifting and/or a decrease in mass resolution.
U.S. Pat. No. 6,087,658 discloses addressing this problem by modifying the hyperbolic surface by constructing a bulge around the internal end of each aperture. The bulge is intended to correct the deviation of the pure quadrupole field caused by the holes (e.g. apertures) in the end caps. It is technically difficult to add such a bulge to an ideal hyperbolic surface. Further, once the surface is modified to include the bulge, the distribution of the quadrupole potential is determined and there is typically no convenient way to change or adjust it.
U.S. Pat. No. 6,608,303 discloses the use of an aperture shim electrode placed in the aperture to correct the deviation. A shim lens power supply provides a different RF voltage for the shim electrode than for the primary electrode which permits a correction of the quadrupole field deviation caused by the presence of the aperture. A shim electrode with an additional power supply provides the possibility of altering the potential distribution including altering the potential distribution as the ion trap is used. However, placement of a shim electrode in the aperture is limited by the aperture size. Also, the shim electrode affects potential distribution in the region immediately around the aperture but has less influence elsewhere.
U.S. Pat. No. 5,650,617 also describes using an aperture shim electrode in the aperture to improve ion trapping for the externally produced ions.
U.S. Pat. No. 5,468,958 describes a sectional ion trap composed of multiple rings of cylindrical symmetry to introduce higher order multiple fields which can be tuned electronically. The sectional ion trap has the disadvantage of being difficult to make. Further, it is typically difficult to maintain the correct geometry between its sectional electrodes.
Accordingly, there is a need for reducing the deviations in the quadrupole field.